It is often necessary in digital circuitry to generate a pulse of defined length in response to a pulse of undefined length. This is frequently done with a delay line loop and a two input logic gate, for example, a NAND or a NOR gate.
In such a typical circuit, the circuit input receives the undefined pulse. It is coupled both to one of the inputs of the logic gate and to the delay line. The delay line typically uses either a long odd numbered string of inverters or a capacitor based delay circuit. The other end, or output end of the delay line, is connected to the other input of the logic gate. Assuming the circuit input is normally low and a NAND gate is used, when the circuit input goes high, it will cause the NAND gate output to go low, and it will cause a delay to propagate through the delay line towards the other input of the NAND gate. When the delay reaches the NAND gate, it causes the NAND gate to go high again.
In this way, the circuit converts the long pulse into a short pulse or one shot. The length of the short pulse is determined by the built-in time delay in the delay line. If only inverters are used, then the delay is determined by the time it takes for the signals to propagate through the usually semiconductor materials from which the inverters are made. If the delay line is capacitor based, then the delay is typically determined by the time constant of the resistor capacitor circuit.
While these prior circuits effectively generate one-shot pulses, they do have some drawbacks. First, the electronic circuitry, which provides the input to the one-shot circuit, can provide a variety of different inputs. Some are legitimate signals and some are short glitches or erroneous signals. Prior circuits have not been able to distinguish between legitimate and erroneous signals. In addition, many prior circuits use both active and passive elements. Normally, the passive elements are capacitors. When the circuit is subject to changing environmental conditions and changing loads, the passive and active elements respond differently. As a result, the parameters of the circuit change and its behavior is unreliable.